The present invention relates to an automatic frequency pulling circuit, and more particularly to an automatic frequency pulling circuit capable of producing a signal in synchronism with a horizontal synchronizing signal from a video signal processing device.
Composite video signals produced from video signal processing devices such as CT scanners include a vertical synchronizing signal with serrated pulses inserted at 1/2H intervals. If such serrated pulses were not inserted or were out of proper timing, a video image displayed on a video monitor in response to a composite video signal from the video signal processing device would be disturbed. With the interlacing scanning system, scanning lines in even fields would not be positioned midway between scanning lines in odd fields, resulting in poor quality of displayed images.
There is known an image recording apparatus in which video signals from a video signal processing device used for diagnosis of diseases are displayed on a video monitor and displayed images are photographed. If serrated pulses were not present or were not in proper timing, the recorded images would be disturbed and fail to represent accurate information of the patent, leading to a wrong diagnosis.
In the above image recording apparatus, raster erasure is generally carried out to make image scanning lines less visible by inserting new scanning lines between the image scanning lines on the video monitor. However, such scanning lines used for raster erasure would not be inserted at equal intervals if no serrated pulses were present or serrated pulses were not properly timed.